Method for Separating a Cleaned Useful Gas From a Gas Mixture, and Device for Carrying Out Said Method

ABSTRACT

A method is provided for separating a cleaned useful gas from a gas mixture substantially containing carbon dioxide, at least one useful gas, and at least one hazardous substance. The carbon dioxide is condensed, and the liquid carbon dioxide that is enriched with the hazardous substance is separated from the useful gas. The hazardous material is then separated from the liquid carbon dioxide by adsorption, and one part of the cleaned liquid carbon dioxide is fed into the useful gas to absorb hazardous substances still contained in the useful gas.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International Application No. PCT/EP2011/050756, filed Jan. 20, 2011 and claims the benefit thereof. The International Application claims the benefits of German application No. 10 2010 006 102.6 DE filed Jan. 28, 2010. All of the applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to a method for separating cleaned useful gas from a gas mixture substantially containing carbon dioxide, at least one useful gas, and at least one harmful substance.

BACKGROUND OF INVENTION

Owing to what is termed the greenhouse effect and the global rise in temperature associated therewith, the emission of greenhouse gases into the atmosphere is being judged increasingly critically. A considerable portion of the greenhouse effect is ascribed to carbon dioxide (CO₂), which is produced in the burning of fossil fuels. Fossil fuels are burned in large quantities in, for example, power stations and industrial plants.

To counteract the greenhouse effect, the intention in the future is for carbon dioxide to be as far as possible separated from the exhaust gases emitted by power stations and industrial process. To that end, intensive development work is under way aimed at designing plants that will emit as little carbon dioxide as possible or even none at all. The central aim of all concepts is to separate the carbon dioxide from a gas stream as completely as possible and in a highly pure form, if possible with little expenditure of energy, so that gas that is free from carbon dioxide can finally be emitted into the environment after the separating process.

However, apart from the carbon dioxide requiring to be separated, a gas requiring to be treated frequently also contains other undesired substances, meaning harmful ones, whose occurrence depends on the kind of fossil fuel being burned and which have to be removed. Sulfur and mercury and their compounds can be cited as the substances occurring most frequently in terms of quantity. That means that not only carbon dioxide but also harmful substances need to be separated from the gas.

Harmful substances in the exhaust gas produced in coal-fired power stations, for example, such as the cited sulfur or mercury compounds, have hitherto been separated directly in the gaseous phase. For example hydrogen sulfide is separated with the aid of gas scrubbers. That entails ducting the gas stream through an absorber in which a liquid medium absorbs the harmful substances. Liquid absorber media include aqueous alkanolamine solutions, especially aqueous methyl diethanolamine, and, for example, cold methanol, employed in the rectisol process. What, though, is disadvantageous therein is particularly that methods for gas scrubbing are associated with increased energy requirements.

An efficient possibility for separating carbon dioxide and harmful substances from a gas mixture is described in subsequently published German patent DE 10 2009 035 389.5. It is a method according to which the carbon dioxide is condensed out of the gas mixture cryogenically, meaning that the carbon dioxide is separated from the gas mixture as a liquid phase. Harmful substances contained in the gas mixture therein accumulate in the liquid carbon dioxide. The liquid carbon dioxide laden with harmful substances is then fed to an adsorber device in which the harmful substances contained in the liquid carbon dioxide are adsorbed and consequently separated from the liquid carbon dioxide that will appear at the adsorber device's output in a form very largely or completely free from harmful substances.

The resulting useful gas will have been cleansed of CO₂ because of the carbon dioxide's cryogenic separation. It has, though, proved to be the case that not all harmful substances accumulate in the liquid CO₂ phase during the condensing process but, instead, that a certain residual amount of harmful substances will still be contained in the useful gas.

SUMMARY OF INVENTION

The object of the invention is therefore to disclose a method for separating cleaned useful gas from a gas mixture, which useful gas will not only have been very largely cleansed of CO₂ but will also be very largely free from harmful substances.

To achieve said object, in the case of a method for separating cleaned useful gas from a gas mixture substantially containing carbon dioxide, at least one useful gas, and at least one harmful substance it is inventively provided for carbon-dioxide condensing to be carried out and for the liquid carbon dioxide containing an accumulation of harmful substances to be separated from the useful gas, after which the harmful substances will be separated from the liquid carbon dioxide by adsorption and a part of the cleaned liquid carbon dioxide will be fed to the useful gas for absorbing harmful substances still contained in the useful gas.

The inventive method thus likewise provides for the carbon dioxide to be cryogenically condensed out of the gas mixture requiring to be cleaned, after which the liquid carbon dioxide laden with harmful substances will be cleansed of the harmful substances contained therein by means of an adsorption device so that liquid carbon dioxide that is at least largely free from harmful substances will appear at the end of the adsorption process. A part of said cleaned liquid carbon dioxide is then inventively fed back and ducted to the useful gas again. An advantageous result of that repeated mixing of the useful gas with the liquid carbon dioxide is that the residual harmful substances contained in the useful gas will then accumulate in the liquid carbon dioxide which, owing particularly to the cleaning process, is to a large extent unsaturated. The liquid carbon dioxide is then fed to the or another adsorption device again in order also to remove the harmful substances taken up as part of the repeated “reaction” with the useful gas from said liquid carbon-dioxide component.

That means that the liquid carbon dioxide is used twice within the scope of the inventive method as an absorber medium for harmful substances contained in the gas mixture. The first time, harmful substances are absorbed within the scope of the carbon dioxide's liquefaction, so while it is being taken to the liquid phase through cooling by means of a suitable carbon-dioxide-condensing device. The second time, the still liquid carbon dioxide is used as a washing agent within the scope of the liquid carbon-dioxide component's being fed back to the useful gas in order to absorb the harmful-substance residues from the useful gas. That means the liquid carbon dioxide that is fed back is used as, so to speak, a “liquid gas-washing agent”.

Finally, the inventive method consequently furnishes two separate, cleaned phases, namely on the one hand the useful gas free from carbon dioxide and harmful substances and, on the other hand, the liquid carbon dioxide free from harmful substances.

A particularly advantageous development of the invention provides, when it has been fed back, for the liquid carbon dioxide to be injected or sprayed into the useful gas. The surface of the liquid carbon dioxide will be extremely enlarged through being thus nebulized so that a large exchanging surface will be produced between the gas phase, meaning the useful gas, and the liquid phase, meaning the liquid CO₂ droplets. The result is a very high absorption rate for the residual harmful substances in the useful gas, which substances can consequently be virtually completely absorbed and removed. The work method will be a particularly efficient one if separating of the liquid carbon dioxide laden with harmful substances and the second process of adsorbing harmful substances by way of the cleaned liquid carbon dioxide that is fed back take place in the same container used for phase separating and the absorption of harmful substances, for example a column or flash container, or suchlike That means that on the one hand, alongside useful gas the carbon dioxide that has been condensed out and is laden with harmful substances is put into the column, where phase separating, which is to say the separating of useful gas and liquid carbon dioxide-phase, takes place, and that, on the other hand, the cleaned liquid carbon dioxide that has been fed back is also preferably injected or sprayed into the column in order to clean the useful gas completely. The liquid carbon dioxide that was fed back is then taken off along with the carbon dioxide laden with harmful substances that was originally fed in and has been condensed out and thereafter fed to a common adsorption device for separating harmful substances out.

The useful gas that is separated can be any useful gas whose occurrence depends on the fossil fuel being burned. Examples are hydrogen, carbon monoxide, nitrogen, an inert gas, or a mixture of the cited gases.

The harmful substances occurring can also vary in nature depending on the specific fossil fuel used. Harmful substances that frequently occur are sulfur and mercury and their compounds, or mixtures of the cited substances.

Apart from the method, the invention relates also to a device for separating cleaned useful gas from a gas mixture substantially containing carbon dioxide, at least one useful gas, and at least one harmful substance. The inventive device comprises at least one carbon-dioxide-condensing device, at least one container in which phase separating and the absorbing of harmful substances takes place, for example a column or flash container, or suchlike, for separating the useful gas from the liquid carbon dioxide laden with harmful substances, at least one adsorption device for taking up the harmful substances contained in the separated liquid carbon dioxide, and a feedback line for feeding back a part of the cleaned liquid carbon dioxide, preferably into the column, for taking up residual harmful substances contained in the useful gas. A device of such kind can be integrated in, for example, a power station or any industrial plant and used there for gas cleaning. The gas mixture that arises is all fed to the carbon-dioxide-condensing device, possibly after being subjected to a suitable pre-treatment. CO₂ condensing takes place in the carbon-dioxide-condensing device. The mixture of useful gas and liquid carbon dioxide is then fed to a container 4 that is suitable for gas/liquid separating and in which phase separating takes place. The liquid carbon dioxide laden with harmful substances that is removed from the column is then fed to the adsorption device for separating the harmful substances off by adsorption. The cleaned liquid carbon dioxide taken from the adsorption device is then in part fed back over a feedback line and brought into contact again with the useful gas already separated from the liquid carbon dioxide in order to remove the residual harmful substances from said gas. The feedback line can therein be ducted directly into the container, meaning that the carbon dioxide that has been fed back is again conveyed into the container to the region already containing the separated useful gas where “useful-gas washing” takes place by means of the liquid carbon dioxide acting as an absorption agent. As an alternative to being fed back into the container, cleaned liquid carbon dioxide can of course also be combined with the useful gas in a suitable “reaction vessel” outside the column for useful-gas washing to be performed there. The device's operating temperature is in the range of −0° C. to −10° C., preferably −60° C. to −30° C.

In a development of the invention an injecting or spraying device is provided for injecting or spraying the liquid carbon dioxide into the container or reactor in which the harmful substances are absorbed from the useful gas, thus in the column itself, for example. The exchanging surface, meaning the surface of the liquid CO₂ phase, can be set or changed by way of the degree of nebulizing by means of the injecting or spraying device, with a large exchanging surface being expedient for absorbing the residual harmful substances highly efficiently.

If the injecting or spraying device is located in the container itself, then it will be positioned expediently underneath a demister that is located in the column and through which the useful gas flows but via which fluid droplets of the liquid CO₂ are kept back.

The adsorption device itself is preferably a stationary bed adsorber. For example active carbon, zeolite, alum earth (aluminum oxide), a polymer having a large surface, or suchlike is employed as the adsorber medium (adsorbent), with an adsorber medium of such kind that can be very easily regenerated preferably being used. Continuous operation can therein be ensured particularly if a plurality of adsorption devices have been provided that can be switched over in turns between an adsorption mode and a regeneration mode. That will enable a part of the adsorption devices to be operated continuously in the adsorption mode, and hence CO₂ cleaning to be carried out continuously, and the other part of the adsorption devices to be regenerated. Switchover between the two operating modes is performed as and when necessary so that a specific number of adsorption devices will always be operated in the adsorption mode.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages, features, and specifics of the invention will emerge from the exemplary embodiment described below and with the aid of the drawing.

DETAILED DESCRIPTION OF INVENTION

The FIGURE shows an inventive device 1 for separating a cleaned useful gas from a gas mixture 2 occurring as a gas stream, for example a stream of exhaust gas, for instance in a power station or other industrial plant in which fossil fuels are used. Gas stream 2 contains carbon dioxide as the mixture's main constituent as well as at least one useful gas such as hydrogen, for instance, and at least one harmful substance based on, for example, sulfur or mercury, for example hydrogen sulfide (H₂S) or carbonyl sulfides (COS). Exhaust gas 2 is fed as a gas stream first to a carbon-dioxide-condensing device 3 in which exhaust gas 2 is cooled to a temperature at which the carbon dioxide present therein previously in gaseous form will condense. It will therefore go to a liquid phase. The harmful substances originally contained in exhaust gas 2 are largely, though not completely, dissolved in the liquid CO₂ phase. Both phases, which is to say the useful gas still laden with residual harmful substances and the liquid carbon dioxide laden with harmful substances, are then taken to a container 4, for example a column 4, where phase separating takes place. Liquid carbon dioxide 5 laden with harmful substances is deposited in the container's lower region; in the column's upper region or, as the case may be, above CO₂ level, is the useful gas 6 which, via a demister 7 serving to hold back fluid CO₂ droplets still contained in the gas, is taken off as useful-gas stream 8.

As described, liquid carbon dioxide 5 accumulates in the column's lower region. Depending on the operating temperature of inventive device 1 or, as the case may be, carbon-dioxide-condensing device 3, liquid carbon dioxide 5 may also contain solid carbon dioxide in particle form. The operating temperature should, though, be such as to preclude a third, solid phase so that possible inclusion of the adsorption devices described below will be obviated.

Liquid carbon dioxide 5 augmented with harmful substances is taken off and fed to at least one, in the example shown three adsorption devices 9 in which the harmful substances contained in liquid carbon dioxide 5 are removed by adsorption. As described, the harmful substances contained in the gas mixture are automatically substantially augmented during carbon-dioxide condensing in the liquid carbon-dioxide phase. That now makes simple separating possible in adsorption devices 9 by means of adsorption. Adsorption devices 9 are preferably stationary bed adsorbers into which the liquid carbon-dioxide stream is fed along with the harmful substances. For example active carbon, zeolite, alum earth, or a polymer can be used as the adsorber medium depending on the specific harmful substance or mixtures of harmful substances requiring to be adsorbed.

Three adsorber devices 9 have been connected in parallel in the example shown. That offers the possibility of operating only a part, for example two adsorption devices 9, simultaneously in the adsorption mode while the third adsorption device 9 operates in the regeneration mode in order to regenerate the adsorber medium, meaning to remove the harmful substances adsorbed thereon and desaturate the adsorber medium again. Appropriate lines and valves enabling a switchover mode are provided for that purpose in a manner that is known per se but not presented in further detail.

Cleaned liquid carbon dioxide 10 that hence now contains no or virtually no harmful substances is then taken from the adsorption devices 9.

Over a feedback line 11 by means of a pump 12, a part of the cleaned liquid carbon dioxide is then conveyed back again to column 4 and nebulized there as finely as possible by means of an injecting or spraying device 13. Injecting or spraying device 13 is located close to demister 7 in a region underneath it, which is to say in the column region containing useful gas 6. Injecting or spraying cleaned carbon dioxide 10 results in a further absorption reaction between useful gas 6 or, as the case may be, the harmful substances contained therein and the fine CO₂ droplets, so the latter will absorb the residual harmful substances. Cleaned useful gas 6 flowing through demister 7 and then taken off will hence be free not only from carbon dioxide but also from harmful substances. Sprayed carbon dioxide 10, which will then likewise or again be laden with harmful substances, will then be taken off with liquefied carbon dioxide 5 that was fed in originally and is located in the column's lower region and fed to adsorption devices 9 again.

The portion of cleaned carbon dioxide 10 not taken off via feedback line 11 is fed to any kind of suitable post-processing operation.

As described, the maximum temperature for the method should be −30° C.; to prevent the carbon dioxide from solidifying it should not be below −60° C. The pressure for the method must basically be above the triple point of carbon dioxide in keeping with the temperature/pressure diagram. It is accordingly at least 5 bar. The relevant operating parameters and the adsorber media employed must of course be used in conformity with the relevant gas-mixture constituents or, as the case may be, harmful substances. 

1-11. (canceled)
 12. A method for separating a cleaned useful gas from a gas mixture substantially comprising carbon dioxide, a useful gas, and a harmful substance, the method comprising: carrying out carbon dioxide condensing; separating liquid carbon dioxide containing an accumulation of harmful substances from the useful gas; separating the harmful substances from the liquid carbon dioxide by adsorption; and feeding a part of the cleaned liquid carbon dioxide to the useful gas for absorbing harmful substances still contained in the useful gas.
 13. The method as claimed in claim 12, wherein the liquid carbon dioxide is injected or sprayed into the useful gas.
 14. The method as claimed in claim 12, wherein the liquid carbon dioxide laden with harmful substances is separated from the useful gas in a container into which the cleaned liquid carbon dioxide is also fed.
 15. The method as claimed in claim 12, wherein hydrogen, carbon monoxide, nitrogen, an inert gas, or a mixture thereof is separated as the useful gas.
 16. The method as claimed in claim 12, wherein sulfur, mercury, their compounds, or mixtures thereof are separated as harmful substances.
 17. A device for separating a cleaned useful gas from a gas mixture substantially containing carbon dioxide, a useful gas, and a harmful substance, the device comprising: a carbon dioxide condensing device; a container for separating the useful gas from the liquid carbon dioxide laden with harmful substances; an adsorption device for taking up the harmful substances contained in the separated liquid carbon dioxide; and a feedback line for feeding back a part of the cleaned liquid carbon dioxide, into the container, for taking up residual harmful substances contained in the useful gas.
 18. The device as claimed in claim 17, wherein an injecting or spraying device for injecting or spraying the liquid carbon dioxide is disposed in the container.
 19. The device as claimed in claim 18, wherein the injecting or spraying device is provided underneath a demister that is disposed in the container.
 20. The device as claimed in claim 17, wherein the container for separating the gaseous and liquid phases is a column or flash container.
 21. The device as claimed in claim 17, wherein the adsorption device is a stationary bed adsorber.
 22. The device as claimed in claim 17, wherein a plurality of adsorption devices have been provided that are switched over in turns between an adsorption mode and a regeneration mode. 